1. Field of the Invention
This invention relates to dielectric ceramics for high frequencies, which indicates a particularly high dielectric constant and high Q value at micro-wave frequencies.
2. Description of the Related Art
The dielectric ceramics for high frequency has a wide range of applications in the fields of portable wireless apparatuses, satellite broadcasting, micro-wave integrated circuits, etc. Above all, these dielectric ceramics are used as the dielectric resonators for improving the filter characteristics and frequency stability, which accelerate reduction in size of the instruments and circuits used in the above applications. The following characteristics of the dielectric ceramics are required when it is used in the microwave region: (1) a high dielectric constant for the reduction in size of the resonator, since the size of the dielectric resonator is in inverse proportion to the square root of the dielectric constant; (2) a low dielectric loss; and (3) an excellent temperature stability of resonant frequency.
As well known dielectric ceramics having such characteristics as mentioned above, there can be exemplified a compound as disclosed in, for example, Japanese Patent Laid-Open No. 198505/1986 to be represented by the following general formula: aSrO.multidot.bCaO.multidot.cTiO2.multidot.dSnO2 (where 0.39.ltoreq.a.ltoreq.0.70, 0.ltoreq.b.ltoreq.0.28, 0.27.ltoreq.c.ltoreq.0.35, 0.ltoreq.d.ltoreq.0.04, a+b+c+d=1), so forth.
However, it is generally very difficult to attain both a high relative dielectric constant and a low dielectric loss. To be precise, the relative dielectric constant of the above mentioned conventional dielectric ceramics is about 40, and Q value (1/dielectric loss) thereof indicates a range from several thousands to 15,000 or so. While such dielectric ceramics exists in a comparatively large quantity, the ceramics materials having relative high dielectric constant are recognized to have a tendency to increase their dielectric loss, remarkably. The current situation is that those ceramics materials having their relative dielectric constant of 80 and indicating the Q value of 1,000 at the same measuring frequency, as mentioned above, are available only in a very small quantity.